1. Field of the Invention
This invention relates generally to a small electric motor, and more specifically to a small electric motor wherein a commutator surface comprising commutator arcuate segments and gaps interposed between the commutator pieces at equal intervals is coated with conductive grease to reduce mechanical friction by reducing the pressure of brushes on the commutator surface as well as to reduce spark generation.
2. Description of the Prior Art
In general, brushes of a small electric motor have been adapted to exert a relatively large pushing force on a commutator by means of elasticity of the brushes themselves or other external resilient materials so that the sliding portions of the brushes are not unwantedly skipped at the contact points with the commutator. However, in a state where the brush sliding portions are pushed onto the commutator with a large force, unwanted friction loss is caused on the motor commutator and the brushes themselves tend to be rapidly worn out. On the other hand, if the pushing force is reduced, the brushes tend to skip, as described above, increasing electrical wear due to sparks generated by poor electrical contact. In selecting the brush pressure, therefore, the optimum range of the brush pressure is usually determined, taking into account conflicting requirements to prevent mechanical wear and electrical wear, as described above. In a small electric motor using metal brushes, the commutator surface is often coated with grease to prevent the unwanted skipping of the brushes while minimizing the abovementioned mechanical wear. In such a case, edges are provided on the brushes to cut the dielectric grease film to prevent decrease in conductivity between the brushes and the commutator caused by the presence of grease film.
As described above, grease coating is an effective method, but it requires sufficient brush pressure to cause the edges to cut the grease film. This cannot necessarily reduce the abovementioned mechanical wear at the sliding portions where the brushes contact the commutator surface.